Especially in the pulp and paper field, it is highly desirable to be able to pump suspensions having a medium consistency, e.g. about 8-15% solids. In the pulp and paper art, this is typically accomplished utilizing a degassing pump having an impeller of a centrifugal pump rotating at a speed sufficient to effect fluidization. Such a degassing pump is shown by U.S. Pat. No. 4,435,193. While such pumps are successful, there are many installations today where it is difficult or impractical to retrofit such a pump.
According to the present invention, a pumping system and method are provided that allow an ordinary paper pulp stock pump, designed for handling stock of about 3-8% consistency, to pump medium consistency pulp as well. The system according to the invention may be retrofit, although under some circumstances it is even desirable to provide a complete system according to the invention in new installations. The pumping system according to the invention does not in any way affect the mounting, speed of operation, or any other parameters of the ordinary stock pump, but merely provides additional elements that are not connected to, although they operatively interact with, the ordinary centrifugal stock pump, to alloW it to pump medium consistency suspensions.
According to one aspect of the present invention, a pumping system for suspensions is provided which comprises the following elements: (a) A centrifugal pump having a housing containing an impeller, the impeller having a hub and blades. (b) Means for rotating the impeller about an impeller axis, in a first direction of rotation. (c) Means defining an inlet to the housing, and an outlet from the housing, for suspension being pumped. (d) A vaned tubular element having an open end. (e) Means for mounting the tubular element for rotation about an axis generally in line with the impeller axis, and so that the open end extends through the inlet so that it is positioned adjacent, but spaced from, the hub. And, (f) means for rotating the tube about its axis in a second direction of rotation, opposite the first direction.
According to another aspect of the present invention, a pumping system is provided specifically for pumping suspensions having a solids content of about 8-15%. The pumping system comprises: (a) A centrifugal pump having a housing containing an impeller, the impeller having a hub and blades. (b) Means for rotating the impeller about an impeller axis, in a first direction of rotation. (c) Means defining an inlet to the housing, and an outlet from the housing, for suspension being pumped. (d) A vaned tubular element having an open end. (e) Means for mounting the tubular element for rotation about an axis generally in line with the impeller axis, and so that the open end extends through the inlet so that it is positioned adjacent, but spaced from, the hub. (f) Means for rotating the tube about its axis at a velocity sufficient to fluidize the suspension of consistency about 8-15%. (g) The means defining the inlet comprising a throat, the throat, tube, and vanes on the tube dimensioned so that there is a narrow clearance between the vanes and the throat to facilitate fluidization of the suspension by the tube. The means (e) and (f) comprise a shaft with a gas passage operatively communicating with the hollow interior of the tube, and (h) means are provided for withdrawing gas from the tube and gas passage. The means (h) may include a vacuum pump.
Preferably the centrifugal pump consists essentially of the housing and impeller, being devoid of gas withdrawal means. Also, the system is ideally mounted in a generally vertically disposed vessel for containing the suspension, the pump being disposed adjacent the bottom of the vessel and all of the elements of the pump system being mounted so that the impeller axis is generally horizontal.
According to a method of the present invention, it is possible to pump a suspension of cellulosic fibrous material (paper pulp) having a consistency of about 8-15% utilizing the apparatus earlier described. The method comprises the steps of: (a) Feeding the cellulosic fibrous material suspension at a consistency of 8-15% toward the inlet to the pump housing. (b) Rotating the tube at a velocity sufficient to cause the suspension to be fluidized and to be forced toWard the pump impeller. (c) Rotating the pump impeller to effect discharge of the suspension from the pump outlet. And, the further step (d) of withdrawing gas from adjacent the impeller. Step (d) preferably consists essentially of the step of causing gas to flow through the tube to be discharged at a position remote from the impeller, and steps (b) and (c) are practiced to cause the impeller to rotate in the direction opposite the direction of rotation of the tube.
According to yet another aspect of the present invention there is provided a method of retrofitting a centrifugal pumping system capable of handling cellulosic fibrous material suspension having a consistency of about 3-8%, so that it is capable of pumping a suspension having a consistency of up to about 15%. The method comprises the steps of: (a) Mounting a vaned hollow tube so that it extends through the inlet to the centrifugal pump, with an open end of the tube mounted adjacent, but spaced from, the hub of the impeller, and with the tube vanes having a small clearance between them and the inlet to the housing, and mounting the tube so that it is rotatable about an axis generally coincident with the impeller axis. (b) Rotating the tube in a direction of rotation opposite to the direction of rotation of the impeller, and at a speed sufficient to fluidize pulp suspension having a consistency of up to about 15% and to force the suspension toward the pump impeller. And, (c) withdrawing gas that may collect at the impeller through the tube to be discharged from the pumping system.
It is the primary object of the present invention to provide a system and method for pumping medium consistency paper pulp suspensions, or the like, utilizing an ordinary centrifugal stock pump. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.